Wordline negative boost write-assist circuits for memory bit cells employing a P-type field-effect transistor (PFET) write port(s), and related systems and methods

ABSTRACT

Write-assist circuits for memory bit cells (“bit cells”) employing a P-type Field-Effect transistor (PFET) write port(s) are disclosed. Related methods and systems are also disclosed. It has been observed that as node technology is scaled down in size, PFET drive current (i.e., drive strength) exceeds N-type Field-Effect transistor (NFET) drive current for like-dimensioned FETs. In this regard, in one aspect, it is desired to provide bit cells having PFET write ports, as opposed to NFET write ports, to reduce memory write times to the bit cells, and thus improve memory performance. To mitigate a write contention that could otherwise occur when writing data to bit cells, a write-assist circuit provided in the form of negative wordline boost circuit can be employed to strengthen a PFET access transistor in a memory bit cell having a PFET write port(s).

PRIORITY APPLICATION

The present application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application Ser. No. 62/119,763 filed on Feb. 23,2015 and entitled “WRITE-ASSIST CIRCUITS FOR MEMORY BIT CELLS EMPLOYINGA P-TYPE FIELD-EFFECT TRANSISTOR (PFET) WRITE PORT(S), AND RELATEDSYSTEMS AND METHODS,” which is incorporated herein by reference in itsentirety.

BACKGROUND

I. Field of the Disclosure

The technology of the disclosure relates generally to memory systemsemploying addressable static memory bit cells for reading and writingdata, and more particularly to write-assist circuits for mitigatingwrite contention conditions when writing to bit cells.

II. Background

Supply voltage (i.e., Vdd) scaling is an effective technique formaximizing processor energy efficiency across all market segments,ranging from small, embedded cores in a system-on-a-chip (SoC) to largemulticore servers. As supply voltage in processor-based systems isreduced to conserve power, circuit delay sensitivity to parametervariations amplifies, eventually resulting in circuit failures. Thesecircuit failures limit the minimum operating supply voltage and themaximum energy efficiency of processor-based systems. In currentprocessor-based system designs, static random-access memory (SRAM)caches and/or register files can limit the minimum operation supplyvoltage. SRAM caches and register file bit cells employ nearminimum-sized transistors to maximize capacity. Since uncorrelatedparameter variations (e.g., random-dopant fluctuations, line-edgeroughness) are inversely proportional to the square-root of thetransistor gate area, wide differences exist for the memory bit cellminimum operating voltage to read, write, and retain data.

In this regard, FIG. 1 is a schematic diagram of an exemplary SRAMsystem 100 employing memory bit cells 102(0)(0)-102(M)(N) (“bit cells102(0)(0)-102(M)(N)) for storing data in a data array 104. The dataarray 104 is organized as having ‘N+1’ bit cell columns and ‘M+1’ bitcell rows of bit cells 102 supporting an “N+1” bit wide data word. Abitline driver 112(0)-112(N) is provided for each bit cell column 0-N todrive a selected bitline 114(0)-114(N) and a complement bitline(bitline_b) 114′(0)-114′(N) for read and write operations. A wordlinedriver 108(0)-108(M) is provided for each bit cell row 0-M in the dataarray 104 to control access to the addressed bit cells 102( )(0)-102()(N) in a given bit cell row 0-M based on an index(0)-index(M) decodedfrom a memory address indicating the bit cell row 0-M to be selected. Aclock signal (clk) 110 controls the timing of asserting an activatedwordline 106(0)-106(M) to access a row of bit cells 102( )(0)-102( )(N)in the selected bit cell row 0-M. In a write operation, data bits 0-N tobe written are provided to respective bitline drivers 112(0)-112(N) todrive the received data bits 0-N and their complement data bits onto thebitlines 114(0)-114(N) and complement bitlines 114′(0)-114′(N),respectively. The wordline driver 108(0)-108(M) for the selected bitcell row 0-M is activated to select the memory bit cells 102( )(0)-102()(N) to be written. The data bits 0-N asserted on the bitlines114(0)-114(N) and complement bitlines 114′(0)-114′(N), respectively, arewritten into the selected memory bit cells 102( )(0)-102( )(N).

FIG. 2 is a circuit diagram of a bit cell 102 in the SRAM system 100 inFIG. 1. In this example, the bit cell 102 is a standard six (6)transistor (6-T) static complement memory bit cell. The bit cell 102comprises two (2) cross-coupled inverters 120(0), 120(1) powered byvoltage (Vdd). The cross-coupled inverters 120(0), 120(1) reinforce eachother to retain data in the form of a voltage on a respective truestorage node (T) 122 and a complement storage node (C) 122′. Eachinverter 120(0), 120(1) is comprised of a respective pull-up P-typeField-Effect transistor (PFET) 124(0), 124(1) coupled in series to arespective pull-down N-type Field-Effect transistor (NFET) 126(0),126(1). NFET access transistors 128(0), 128(1) are coupled to therespective inverters 120(0), 120(1) to provide respective read/writeports 130(0), 130(1) to the bit cell 102. In a read operation, thebitline 114 and complement bitline 114′ are pre-charged to voltage(Vdd). Then, the wordline 106 coupled to gates (G) of the NFET accesstransistors 128(0), 128(1) is asserted to evaluate the differentialvoltages on the true storage node 122 and complement storage node 122′to read the bit cell 102. If a logic high voltage level (i.e., a ‘1’) isstored at the true storage node 122 (T=1) and a logic low voltage level(i.e., ‘0’) is stored at the complement storage node 122′ (C=0),assertion of the wordline 106 will cause the NFET access transistor128(1) to discharge the pre-charged voltage on the complement bitline114′ to the complement storage node 122′ and through the NFET 126(1) toground. However, if the NFET access transistor 128(1) is a faster devicethan the PFET 124(1), the discharge of the pre-charged voltage on thecomplement bitline 114′ can cause a charge build up on complementstorage node 122′ that can cause inverter 120(0) to flip the voltage onthe true storage node 122 from a logic ‘1’ to a logic ‘0’, which maycause a subsequent read operation to the bit cell 102 to returnerroneous data. This is known as a read disturb condition.

To mitigate or avoid a read disturb condition from occurring in the bitcell 102 in FIG. 2, the NFET access transistors 128(0), 128(1) could beweakened and the PFETs 124(0), 124(1) in the inverters 120(0), 120(1) bestrengthened. However, this can cause write contention issues in the bitcell 102. FIG. 3 is a circuit diagram illustrating a write contentionbetween the NFET access transistor 128(0) and the PFET 124(0) in theinverter 120(0) in the bit cell 102 in FIG. 2. For example, during awrite operation, if a logic ‘1’ is stored in the true storage node 122(T=1) (and a logic ‘0’ is stored in the complement storage node 122′(C=0)), and the data placed on the bitline 114 to be written to the truestorage node 122 is a logic ‘0’, the NFET access transistor 128(0)discharges the true storage node 122 to the bitline 114 to write a logic‘0’ to the true storage node 122. The NFET access transistor 128(0) iscapable of passing a strong logic ‘0’. However, the logic ‘0’ stored inthe complement storage node 122′ can cause the strengthened PFET 124(0)to overcome the drive strength of the NFET access transistor 128(0) tocharge the true storage node 122 to voltage (Vdd) (i.e., a logic ‘1’),thus causing a write contention on the true storage node 122.

SUMMARY OF THE DISCLOSURE

Aspects of the disclosure involve write-assist circuits for memory bitcells (“bit cells”) employing a P-type Field-Effect transistor (PFET)write port(s). Related methods and systems are also disclosed. The bitcells are provided in a data array of a memory system in aprocessor-based system to store data. It has been observed that as nodetechnology is scaled down in size, PFET drive current (i.e., drivestrength) exceeds N-type Field-Effect transistor (NFET) drive currentfor like-dimensioned FETs. This is due to the introduction of strainedsilicon in FET fabrication to reduce the effective mass of chargecarriers. A write port is a performance critical element in a bit cell.In this regard, in one aspect, it is desired to provide bit cells havingPFET write ports, as opposed to NFET write ports, to reduce memory writetimes to the bit cells, and thus improve memory performance. To mitigatea write contention that could otherwise occur when writing data to bitcells, write-assist circuits are provided for bit cells having a PFETwrite port(s). By mitigating or avoiding write contention issues in thebit cells, the voltage supply providing the minimum voltage to the bitcells for operation and data retention can be reduced to reduce powerconsumption and increase processor energy efficiency.

In this regard, in one aspect, a memory system is provided. The memorysystem comprises a memory bit cell configured to store data in responseto a write operation. The memory bit cell comprises one or more PFETaccess transistors each comprising a gate configured to be activated bya wordline in response to the write operation. The memory system alsocomprises a wordline negative boost circuit coupled to the wordline. Thewordline negative boost circuit is configured to negatively boost avoltage on the wordline to negatively boost a voltage on the one or moregates of the one or more PFET access transistors in response to thewrite operation.

In another aspect, a memory system is provided. The memory systemcomprises a means for storing data for a write operation. The means forstoring data comprises one or more PFET-based access means eachcomprising a gate for being activated by a wordline in response to thewrite operation. The memory system also comprises a means for negativelyboosting a voltage on the wordline in response to the write operation.

In another aspect, a method of writing data to a memory bit cell isprovided. The method comprises charging at least one bitline coupled toan access node of one or more PFET access transistors in a memory bitcell with data in response to a write operation. The method alsocomprises activating a wordline coupled to a gate of the one or morePFET access transistors in response to the write operation to transferthe data from an access node to a storage circuit. The method alsocomprises negatively boosting a voltage on the wordline in response tothe write operation to assist transferring the data from the access nodeto the storage circuit in response to the write operation.

In another aspect, a non-transitory computer-readable medium havingstored thereon computer data for an integrated circuit (IC) is provided.The integrated circuit comprises a memory bit cell configured to storedata in response to a write operation. The memory bit cell comprises oneor more PFET access transistors each comprising a gate configured to beactivated by a wordline in response to the write operation. The memorysystem comprises a wordline negative boost circuit coupled to thewordline. The wordline negative boost circuit is configured tonegatively boost a voltage on the wordline to negatively boost a voltageon the gate of the one or more PFET access transistors in response tothe write operation.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram of an exemplary processor-based memorysystem employing a static random access memory (SRAM) system comprisinga data array of static memory bit cells organized in rows and columnsfor storing data;

FIG. 2 is a circuit diagram illustrating a read disturb condition in astandard six (6) transistor (6-T) static complement memory bit cell thatcan be employed in the data array in the SRAM system in FIG. 1;

FIG. 3 is a circuit diagram illustrating a write contention between anN-type Field-Effect transistor (NFET) access transistor and an inverterP-type Field-Effect transistor (PFET) in a standard 6-T staticcomplement memory bit cell that can be employed in the data array in theSRAM system in FIG. 1;

FIG. 4 is a graph illustrating relative saturation drain currents(I_(DSAT)) of NFET and PFET technology as a function of technology nodesize;

FIG. 5 is a schematic diagram of an exemplary processor-based memorysystem employing an SRAM system comprising a data array of static memorybit cells employing PFET write ports;

FIG. 6A is a circuit diagram illustrating an exemplary write conditionin a standard 6-T static complement bit cell employing a PFET write portin the data array in the SRAM system in FIG. 5;

FIG. 6B is a flowchart illustrating exemplary processes for providingwrite assistance for the memory bit cell including PFET accesstransistors in FIG. 6A, to avoid write contentions and assist intransferring data into a storage circuit in the bit cell in response toa write operation;

FIG. 7 illustrates an exemplary write-assist circuit that includes awordline negative boost circuit provided in a wordline driver tonegatively boost a wordline of the memory bit cells in FIG. 5 tostrengthen PFET access transistors in the memory bit cells in responseto a write operation to avoid write contentions;

FIG. 8 illustrates another exemplary write-assist circuit that includesa wordline negative boost circuit provided in a wordline driver tonegatively boost a wordline of the bit cells in FIG. 5, whichadditionally includes charge path selection circuits to build a negativeboost charge without a charge setup time required and reducedcapacitance on the wordline when the write-assist circuit is disabled,to strengthen PFET access transistors in the bit cells in response to awrite operation to minimize write contentions;

FIG. 9 illustrates an exemplary write-assist circuit that includes abitline positive boost circuit provided in a bitline driver topositively boost bitlines of the bit cells in FIG. 5 to strengthen PFETaccess transistors in the bit cells in response to a write operation toavoid write contentions;

FIG. 10 illustrates another exemplary write-assist circuit that includesa bitline positive boost circuit provided in a bitline driver topositively boost bitlines of the bit cells in FIG. 5 to strengthen PFETaccess transistors in the bit cells in response to a write operation toavoid write contentions;

FIG. 11 illustrates an exemplary write-assist circuit that includes anexemplary negative supply rail positive boost circuit configured toweaken an NFET in cross-coupled inverters in the bit cells in FIG. 5 tobe weaker than PFET access transistors in the bit cells in response to awrite operation to avoid write contentions;

FIG. 12 illustrates another exemplary write-assist circuit that includesanother exemplary negative supply rail positive boost circuit configuredto weaken an NFET in cross-coupled inverters in the bit cells in FIG. 5to be weaker than PFET access transistors in the bit cells in responseto a write operation to avoid write contentions; and

FIG. 13 is a block diagram of an exemplary processor-based system thatcan include memory systems that employ write-assist circuits for memorybit cells with PFET write ports, and according to any of the aspectsdisclosed herein.

DETAILED DESCRIPTION

With reference now to the drawing figures, several exemplary aspects ofthe present disclosure are described. The word “exemplary” is usedherein to mean “serving as an example, instance, or illustration.” Anyaspect described herein as “exemplary” is not necessarily to beconstrued as preferred or advantageous over other aspects.

Memory bit cells are provided in a data array of a memory system in aprocessor-based system to store data. As shown in a graph 400 in FIG. 4,it has been observed that as node technology is scaled down in size,P-type Field-Effect Transistor (PFET) drive current (i.e., drivestrength) exceeds N-type Field-Effect transistor (NFET) drive currentfor like-dimensioned FETs. This is due to the introduction of strainedsilicon in FET fabrication to reduce the effective mass of chargecarriers. As illustrated in FIG. 4, the technology node size innanometers (nm) is provided on an X-axis 402. The ratio of a saturationdrain current of a PFET (I_(DSAT, N)) of an NFET to the saturation draincurrent (I_(DSAT, N)/I_(DSAT, P)) is provided on a Y-axis 404. The ratioof I_(DSAT, N) to I_(DSAT, P) as a function of technology node size innm is shown on a ratio line 406. As shown by the ratio line 406 in FIG.4, a PFET drive strength increases as compared to a like-dimensionedNFET as the technology node size decreases. At point 408, the ratio line406 crosses a ratio of 1.0 of NFET drive strength to PFET drivestrength. Thus, in this example, the drive strength of the PFET isgreater than the drive strength of a like-dimensioned NFET.

In this regard, as discussed in more detail below, aspects disclosedherein involve write-assist circuits for memory bit cells (“bit cells”)employing a P-type Field-Effect transistor (PFET) write port(s). A writeport may be a performance critical element in a bit cell. In thisregard, in one aspect, it is desired to provide bit cells having PFETwrite ports, as opposed to NFET write ports, to increase memory writetimes to the bit cells, and thus improve memory write performance. Asshown in the saturation drive current (I_(DSAT)) equation below, anincrease in the charge carrier effective mobility results in an increasein drive current (I_(D)).I _(D)=½μC _(ox) W/L(V _(GS) −V _(TH))²

where:

-   -   ‘μ’ is the charge carrier effective mobility,    -   ‘W’ is the gate width,    -   ‘L’ is the gate length,    -   ‘C_(ox)’ is the capacitance of the oxide layer;    -   ‘V_(GS)’ is the gate-to-source Voltage, and    -   ‘V_(TH)’ is the threshold voltage.

In this regard, in one aspect, it is desired to provide bit cells havingPFET write ports, as opposed to NFET write ports, to reduce memory writetimes to the bit cells, and thus improve memory performance. To mitigatea write contention that could otherwise occur when writing data to bitcells, write-assist circuits are provided for bit cells having a PFETwrite port(s). By mitigating or avoiding write contention issues in thebit cells, the voltage supply providing the minimum voltage to the bitcells for operation and data retention can be reduced to reduce powerconsumption and increase processor energy efficiency.

In this regard, FIG. 5 is a schematic diagram of an exemplary SRAMsystem 500 employing PFET write port bit cells 502(0)(0)-502(M)(N) (alsoreferred to herein as “memory bit cells 502(0)(0)-502(M)(N)”). PFETwrite port memory bit cells are bit cells that each have one or morePFET write ports. As discussed above, if the technology node of the bitcells 502(0)(0)-502(M)(N) is scaled down far enough, the bit cells502(0)(0)-502(M)(N) will perform write operations faster than similarbit cells employing NFET write ports. The bit cells 502(0)(0)-502(M)(N)are configured to store data in a data array 504. As non-limitingexamples, the bit cells 502(0)(0)-502(M)(N) may be standard six (6)transistor (6-T) or eight transistor (8-T) static complement memory bitcells. The data array 504 is organized as having ‘N+1’ bit cell columnsand ‘M+1’ bit cell rows of the bit cells 502 supporting an ‘N+1’ bitwide data word. For any given bit cell row 0-M in the data array 504,each bit cell column 0-N of the data array 504 includes a memory bitcell 502 in which a single data value or bit is stored.

With continuing reference to FIG. 5, write operations to the bit cells502(0)(0)-502(M)(N) are controlled by respective wordlines 506(0)-506(M)corresponding to each bit cell row 0-M. A memory write operation isperformed on the bit cells 502( )(0)-502( )(N) in a given bit cell row0-M based on wordline enable signals (wl_enb) 507(0)-507(M) generated asa result of decoding the memory address in a memory write operationindicating the bit cell row 0-M to be written. The wordline enablesignals 507(0)-507(M) may act as wordline enable signals to indicate awrite operation. A wordline driver 508(0)-508(M) is provided for eachbit cell row 0-M in the data array 504 to control writing to theaddressed bit cells 502( )(0)-502( )(N) in a given bit cell row 0-Mbased on the received respective wordline enable signal (wl_enb)507(0)-507(M). Thus, only one wordline driver 508(0)-508(M) is activatedat a time in this example. A write clock signal (write_clk) 510 controlsthe timing of asserting the activated wordline 506(0)-506(M) to data tothe bit cells 502( )(0)-502( )(N) in the selected bit cell row 0-M inresponse to a write operation.

With continuing reference to FIG. 5, a bitline driver 512(0)-512(N) isprovided for each bit cell column 0-N in the SRAM system 500 for writingdata into the bit cells 502( )(0)-502( )(N) in the selected bit cell row0-M. In this regard, the bitline drivers 512(0)-512(N) drive a bitline514(0)-514(N) and a complement bitline (bitline_b) 514′(0)-514′(N),because the bit cells 502(0)(0)-502(M)(N) employ a complement bitlinearchitecture.

In a write operation, data bits 0-N to be written are provided torespective bitline drivers 512(0)-512(N) to drive the received data bits0-N and their complement data bits onto the bitlines 514(0)-514(N) andcomplement bitlines 514′(0)-514′(N), respectively. The wordline driver508(0)-508(M) for the selected bit cell row 0-M is activated to selectthe memory bit cells 502( )(0)-502( )(N) to be written. The data bits0-N asserted on the bitlines 514(0)-514(N) and complement bitlines514′(0)-514′(N), respectively, are written into the selected memory bitcells 502( )(0)-502( )(N).

In a read operation, the bitline drivers 512(0)-512(N) pre-discharge thebitlines 514(0)-514(N) and complement bitlines 514′(0)-514′(N) during apre-discharge stage. The wordline driver 508(0)-508(M) for the selectedbit cell row 0-M causes the data stored in the selected bit cells 502()(0)-502( )(N) to be asserted onto the bitlines 514(0)-514(N) andcomplement bitlines 514′(0)-514′(N) to be sensed by sense amplifiers516(0)-516(N) provided in each bit cell column 0-N. The sense amplifiers516(0)-516(N) provide the read data bits from the selected bit cells502( )(0)-502( )(N) onto respective data output lines 518(0)-518(N).

FIG. 6A is a circuit diagram illustrating a write contention conditionin a PFET write port bit cell 502 employing a PFET read/write port inthe data array 504 in the SRAM system 500 in FIG. 5. The PFET write portbit cell 502 comprises a storage circuit 501 in this example comprisedof two (2) cross-coupled inverters 520(0), 520(1) powered by voltage(Vdd). A true storage node (T) 522 and a complement storage node (C)522′ retain data 532 (i.e., a charge) and complement data 532′ (i.e., acomplement charge to the data 532) in the form of a voltage on therespective true storage node (T) 522 and complement storage node (C)522′. Each inverter 520(0), 520(1) is comprised of a respective pull-upPFET 524(0), 524(1) coupled in series to a respective pull-down NFET526(0), 526(1). Instead of providing NFET access transistors, PFETaccess transistors 528(0), 528(1) are coupled to the respectiveinverters 520(0), 520(1) to provide respective read/write ports 530(0),530(1) to the PFET write port bit cell 502. In a read operation, thebitline 514 and complement bitline 514′ are pre-charged to voltage(Vdd). Then, the wordline (WL) 506 coupled to gates nodes (G) (alsoreferred to herein as “gate”) of the PFET access transistors 528(0),528(1) is asserted to evaluate the differential voltages on the truestorage node 522 and complement storage node 522′ to read the PFET writeport bit cell 502.

FIG. 6B is a flowchart illustrating an exemplary process 600 for FIG. 6Bproviding write-assistance for the memory bit cell 502 including thePFET access transistors 528(0), 528(1) in FIG. 6A in response to writeoperations to avoid write contentions and assist in transferring data532 and/or complement data 532′ into the storage circuit 501. Theprocess 600 in FIG. 6B will be discussed in conjunction with FIG. 6A.

In this regard, in a write operation, the bitline 514 and complementbitline 514′ are pre-set to the desired voltage level to represent thelogic value to be written as data 532 and complement data 532′,respectively, to the storage circuit 501 (block 602 in FIG. 6B). Forexample, assume that prior to a write operation, a logic ‘1’ is storedin the true storage node 522 (i.e., T=1) (and a logic ‘0’ is stored inthe complement storage node 522′ (i.e., C=0)). Assume that the data 532to be written from the bitline 514 to the true storage node 522 is alogic ‘0.” In this regard, a logic ‘1’ is placed on the complementbitline 514′ to be written to the complement storage node 522′ as thecomplement data 532′. When the wordline 506 is activated to provide alow voltage (e.g., GND voltage) to a gate (G) of the PFET accesstransistor 528(0), the PFET access transistor 528(0) will be activatedbased on the gate (G)-to-source (S) voltage (VGs). The PFET accesstransistor 528(0) passes the data 532 from the bitline 514 coupled toits access node 534(0) to the true storage node 522 to discharge thetrue storage node 522′ with the logic ‘0’ (i.e., lower voltage) from thebitline 514 (block 604 in FIG. 6B). Also, when the wordline 506 isactivated to provide a low voltage (e.g., GND voltage) to the gate (G)of the PFET access transistor 528(1), the PFET access transistor 528(1)passes the complement data 532′ from its access node 534(1) to thecomplement storage node 522′ to charge the complement storage node 522′with the logic ‘1’ (i.e., higher voltage) from the complement bitline514′ (block 604 in FIG. 6B). The PFET access transistor 528(1) iscapable of passing a strong logic ‘1’. However, at the same time, thepull-down NFET 526(1) is trying to discharge the true storage node 522to GND. This is because the PFET access transistor 528(0) cannot pass astrong logic ‘0’ to the true storage node 522 to turn off the pull-downNFET 526(1). The pull-down NFET 526(1) will be partially turned on as aresult. This causes the PFET access transistor 528(1) and the pull-downNFET 526(1) to be in write contention to write a voltage to thecomplement storage node 522′, either from the complement bitline 514′ orGND.

To mitigate or avoid write contentions from occurring in the PFET writeport bit cell 502 in FIG. 6A, the PFET access transistors 528(0), 528(1)could be strengthened to increase their gate (G)-to-source (S) (V_(GS))voltage and/or the pull-down NFETs 526(0), 526(1) in the inverters520(0), 520(1) could be weakened (block 606 in FIG. 6B). In this regard,write-assist circuits can be provided for bit cells employing a PFETwrite port(s), including the bit cells 502 in the SRAM system 500 inFIG. 5. There are different exemplary write-assist circuits disclosedherein that can be provided for the memory bit cell 502 to mitigate oravoid write contentions in response to write operations without creatingread disturb conditions. In this manner, as technology node of thememory bit cell 502 is scaled down, the benefit of the faster read timesin the memory bit cell 502 employing the PFET write ports 530(0), 530(1)can be realized while mitigating or avoiding write contentionconditions. By mitigating or avoiding write contentions in the memorybit cell 502, the voltage supply providing the minimum voltage (Vdd) tothe memory bit cell 502 for operation and data retention can be reducedto reduce power consumption in the SRAM system 500.

In this regard, as another example, to mitigate or avoid writecontentions in the PFET write port bit cell 502, a write-assist circuitemploying a negative wordline boost circuit configured to negativelyboost the voltage of the wordline 506 in response to a write operationon the PFET write port bit cell 502 can be employed (block 606A in FIG.6B). Examples of memory systems that include write-assist circuitsemploying a negative wordline boost circuit for a PFET write port bitcell are shown in FIGS. 7 and 8, discussed below.

Also, as another example to mitigate or avoid write contentions in thePFET write port bit cell 502, a write-assist circuit employing apositive bitline boost circuit configured to positively boost thevoltage of the bitline 514 in response to a write operation on the PFETwrite port bit cell 502 can be employed (block 606B in FIG. 6B).Examples of memory systems that include write-assist circuits employinga positive bitline boost circuit for a PFET write port bit cell areshown in FIGS. 9 and 10 discussed below.

Also, as another example to mitigate or avoid write contentions in thePFET write port bit cell 502, a write-assist circuit employing anegative supply rail positive boost circuit configured to weaken an NFETpull-down transistor in the storage circuit 501 of the PFET write portbit cell 502 in response to a write operation can be employed (block606C in FIG. 6B). Examples of memory systems that include write-assistcircuits employing a negative supply rail positive boost circuit for aPFET write port bit cell are shown in FIGS. 11 and 12 discussed below.

As discussed above, to mitigate or avoid write contentions in the PFETwrite port bit cell 502, the PFET access transistors 528(0), 528(1) canbe strengthened in response to write operations such that the truestorage node 522 or complement storage node 522′ is not discharged toGND by the respective pull-down NFET 526(0) or 526(1). In this regard,FIG. 7 illustrates an exemplary wordline driver 508A that includes awrite-assist circuit 700 in the form of an exemplary wordline negativeboost circuit 702 to strengthen the PFET access transistors 528(0),528(1) in response to a write operation. The wordline negative boostcircuit 702 in this example is configured to negatively boost thevoltage on the wordline 506 to increase the gate-to-source voltage(V_(GS)) of the PFET access transistors 528(0), 528(1) and thus strengththe PFET access transistors 528(0), 528(1) in response to writeoperations. This mitigates or avoids a write contention between the PFETaccess transistors 528(0), 528(1) and the respective pull-down NFETs526(0), 526(1) in response to write operations. In this manner, thepull-down NFETs 526(0) or 526(1) do not discharge the true storage node522 or complement storage node 522′ to GND when a logical ‘1’ is beingwritten to the true storage node 522 or complement storage node 522′during a write operation. By negatively boosting the voltage on thewordline 506, the drive current (i.e., drive strength) of the PFETaccess transistors 528(0), 528(1) is strengthened according to thefollowing saturation drive current equation by increasing gate-to-sourcevoltage (V_(GS)), as follows:I _(D)=½μC _(ox) W/L(V _(GS) −V _(TH))²

where:

-   -   I_(D) is drive current,    -   ‘μ’ is the charge carrier effective mobility,    -   ‘W’ is the gate width,    -   ‘L’ is the gate length,    -   ‘C_(ox)’ is the capacitance of the oxide layer;    -   ‘V_(GS)’ is the gate to source Voltage, and    -   ‘V_(TH)’ is the threshold voltage.

With continuing reference to FIG. 7, to perform a write operation in thememory bit cell 502 in FIG. 7, the wordline 506 is driven to a logical‘0’ to turn on the PFET access transistors 528(0), 528(1) in the memorybit cell 502. In this regard, the wordline driver 508A in FIG. 7includes a discharge control circuit 703. The discharge control circuit703 is provided in the wordline negative boost circuit 702. The wordlinenegative boost circuit 702 is configured to generate a charge on thewordline (WL) 506 in response to a write operation to negatively boostthe gate-to-source voltage (Vgs) of the PFET access transistors 528(0),528(1). In this regard, the discharge control circuit 703 is configuredto place the wordline (WL) 506 in a floating state to prevent a chargestorage unit 707 in the wordline negative boost circuit 702 fromdischarging a charge stored in the charge storage unit 707 onto thewordline (WL) 506 to negatively boost the voltage on the wordline (WL)506 outside of a write operation. The discharge control circuit 703 inthe example in FIG. 7 includes an NFET 704 coupled to ground (GND). Whena write operation is not enabled, the wordline enable signal 507 is lowor logical ‘0’ in this example. In response, a PFET inside an inverter706 in the discharge control circuit 703 will be turned on to cause theinverter 706 to generate a logical ‘1’ on an output 708 to drive thewordline 506 to logical ‘1’. This causes the PFET access transistors528(0), 528(1) to not be activated.

However, in response to a write operation, the wordline enable signal507 is initially active without the write clock signal 510 being active.A charge control circuit 709 provided in the boost generator circuit 705controls charging of the charge storage unit 707 to store a charge,prior to performing a write operation, to later be discharged tonegatively boost the voltage on the wordline (WL) 506 in response to awrite operation. In this regard, in this example, a charge controlcircuit 709 is provided to control when the boost generator circuit 705charges the charge storage unit 707. An output 710 of a NAND gate 712 inthe charge control circuit 709 is generated as a charge control signal711 and is determined to be a logical ‘1” based on receiving the writeclock signal 510 and the wordline enable signal 507. During this time,the logical ‘1’ output 710 from the NAND gate 712 in the charge controlcircuit 709 will be provided as the charge control signal 711 to aplurality of charge generating circuits 714(0)-714(X) provided in theform of AND gates in this example, in the boost generator circuit 705.Each charge generating circuit 714(0)-714(X) is coupled to the output710 and a respective programmable charge line b(0)-b(x). Thus, theoutput 710 is logical ‘1’ for the charge control signal 711 when theindex is initially enabled during a read operation, if the respectiveprogrammable charge line b(0)-b(x) for the charge generating circuits714(0)-714(X) is also a logical ‘1.’ The charge generating circuits714(0)-714(X) will charge respective charge storage circuits716(0)-716(X) provided in the form of capacitors in this example in thecharge storage unit 707. The respective charge storage circuits716(0)-716(X) will be charged based on the voltage differential betweenthe wordline 506 being driven to GND by the inverter 706 inverting theactive wordline enable signal 507 and the output of the chargegenerating circuits 714(0)-714(X) being Vdd. By providing the pluralityof charge generating circuits 714(0)-714(X) in the boost generatorcircuit 705 of the wordline negative boost circuit 702 that are eachprogrammable via the programmable charge lines b(0)-b(x), the amount ofcharge stored in the charge storage unit 707 to be discharged inresponse to a write operation to negatively boost the wordline (WL) 506can be programmable.

With continuing reference to FIG. 7, when the write clock signal 510transitions to a high signal (logical ‘0’), the output 710 of the NANDgate 712 in the charge control circuit 709 will transition to a logical‘0,’ thereby discontinuing the build-up of charge current in theactivated charge storage circuits 716(0)-716(X) in the charge storageunit 707. The output of the charge generating circuits 714(0)-714(X)will be GND or logical ‘0.” The output 710 of the NAND gate 712 beinglogical ‘0’ will also turn off the NFET 704 in the discharge controlcircuit 703 leaving the wordline (WL) 506 floating. Thus, the chargestored in the charge storage circuits 716(0)-716(X) of the chargestorage unit 707 will be discharged to the wordline (WL) 506. Thus,instead of the wordline (WL) 506 being driven down to GND voltage(logical ‘0’) in this example, the wordline (WL) 506 is driven to anegative voltage below GND to strengthen the PFET access transistors528(0), 528(1) in response to a write operation. This has the effect ofpositively boosting the gate-to-source voltage (V_(GS)) of the PFETaccess transistors 528(0), 528(1) to strengthen the PFET accesstransistors 528(0), 528(1) in response to the write operation.

As discussed in the write-assist circuit 700 in FIG. 7 above, the setuptime between when the wordline enable signal 507 and the write clocksignal 510 are enabled is used to provide a charge time to charge therespective charge storage circuits 716(0)-716(X). It may be desirable toprovide a write-assist circuit to negatively boost the voltage of thewordline 506 of the memory bit cell 502 without the need for a setuptime between when the wordline enable signal 507 and the write clocksignal 510 are enabled In this regard, FIG. 8 illustrates anotherexemplary write-assist circuit 800 provided in a wordline driver 508Bconfigured to negatively boost the wordline 506 of the memory bit cell502 to strengthen the PFET access transistors 528(0), 528(1) during awrite operation to mitigate or avoid a write contention. A wordlinenegative boost circuit 802 in FIG. 8 includes some common componentswith the wordline negative boost circuit 702 in FIG. 7 indicated withcommon element numbers. These common elements will not be re-describedwith regard to FIG. 8. As discussed below, the write-assist circuit 800includes the wordline negative boost circuit 802 to negatively boost thevoltage of the wordline 506 without the need for a charge setup time.Eliminating the charge setup time may improve write operationperformance.

With reference to FIG. 8, the wordline negative boost circuit 802includes a charge path selector 803 comprising a plurality charge pathselection circuits 804(0)-804(X) in this example. Before a writeoperation occurs, the wordline enable signal 507 is non-enabled orlogical ‘0’, and the wordline 506 is at a high or logical ‘1’ such thatthe memory bit cell 502 is not enabled. Thus, the output 710 of the NANDgate 712 in the charge control circuit 709 is logical ‘1.’ The chargegenerating circuits 714(0)-714(X) (AND gates in this example) areenabled by the respective programmable charge lines b(0)-b(x) and willcause voltage (Vdd) to be generated as an output to the charge storagecircuits 716(0)-716(X) in the charge storage unit 707. Also, the output710 of the NAND gate 712 in the charge control circuit 709 will beinverted by an inverter 806 in the charge path selector 803 to generatea logical ‘0’ at a select output (SEL) 808. This will cause NFETs810(0)-810(X) in the respective charge path selection circuits804(0)-804(X) to be turned off such that the charge storage circuits716(0)-716(X) are not connected to the wordline (WL) 506. Instead, theNFETs 812(0)-812(X) in the respective charge path selection circuits804(0)-804(X) are turned on to provide a path to GND for the chargestorage circuits 716(0)-716(X). This allows the charge storage circuits716(0)-716(X) to be charged before the write operation is initiated bythe wordline enable signal 507 being enabled to avoid the charge setuptime in a write operation. When the wordline enable signal 507 and thewrite clock signal 510 are enabled, the output 710 of the NAND gate 712in the charge control circuit 709 is logical ‘0.’ The inverter 806 willgenerate a logical ‘1’ on the select output 808 to turn off NFETs812(0)-812(X) in the respective charge path selection circuits804(0)-804(X) and turn on the NFETs 810(0)-810(X) in the respectivecharge path selection circuits 804(0)-804(X) to discharge the charge inthe charge storage circuits 716(0)-716(X) to the wordline (WL) 506.Thus, the voltage of the wordline (WL) 506 is negatively boosted tostrengthen the PFET access transistors 528(0), 528(1) without regard toa charge setup time in this example of the wordline negative boostcircuit 802. However, additional area may be involved to provide for thecharge path selection circuits 804(0)-804(X).

It is also possible to positively boost the bitline 514 and complementbitline 514′ in lieu of or in combination with negatively boosting thewordline 506 to strengthen the PFET access transistors 528(0), 528(1) inthe memory bit cell 502 in response to write operations to avoid writecontentions. In this regard, FIG. 9 illustrates an exemplarywrite-assist circuit 900 provided in a bitline driver 512A to perform awrite operation in the memory bit cell 502. The write-assist circuit 900includes a bitline positive boost circuit 902 configured to boost thevoltage of the bitline 514 and the complement bitline 514′ of the memorybit cell 502 in FIG. 5 to increase V_(GS) of the PFET access transistors528(0), 528(1) and strengthen the PFET access transistors 528(0), 528(1)during a write operation to avoid write contentions. Note that asdiscussed below, FIG. 9 shows a bitline positive boost circuit 902configured to positively boost the complement bitline 514′ in responseto a write operation. However, another bitline positive boost circuitcould also be provided to positively boost the bitline 514. Either thebitline 514 or complement bitline 514′ is positively boosted in thisexample in response to a write operation. The bitline 514 or complementbitline 514′ that is driven to a logical ‘1’ value in a write operationis the bitline that is positively boosted in response to a writeoperation in this example.

In this regard, with reference to FIG. 9, before a write operation isinitiated by the write clock signal 510, the complement bitline 514′will be in a floating state. More specifically, a charge control circuit913 is provided in the bitline positive boost circuit 902 and configuredto receive the write clock signal 510. The charge control circuit 903 isconfigured to control a complement bitline control circuit 903 to placethe complement bitline 514′ in a floating state when the write clocksignal 510 is not enabled outside of the write operation. This preventsa charge stored in a charge storage circuit 905 from being discharged tothe complement bitline 514′ outside of a write operation. However, whenthe write clock signal 510 is enabled (a logical low voltage level inthis example) for a write operation, the charge control circuit 913causes a charge stored in the charge storage unit 707 to be dischargedto the complement bitline 514′ to positively boost the voltage of thecomplement bitline 514′ in response to a write operation. In thisregard, the charge control circuit 913 generates a charge control signal921 that causes an output 907 of a NAND gate 909 in the complementbitline control circuit 903 in this example to be logical ‘1,’ therebyturning off a PFET 904 and an NFET 906 in the complement bitline controlcircuit 903 to leave the complement bitline 514′ in the floating state.When the write clock signal 510 is not enabled, the charge generatingcircuits 714(0)-714(X) in the boost generator circuit 705 are enabledaccording to their respective programmable charge lines b(0)-b(x) tocharge their respective charge storage circuits 716(0)-716(X) in thecharge storage unit 707. Thus, when the write clock signal 510 is laterenabled for a write operation and complement data 532′ provided on adata input 908 to be written is logical ‘1,’ the NFET 906 in thecomplement bitline control circuit 903 will be turned on, and an output912 of an inverter 914 in the complement bitline control circuit 903will be logical ‘0.’ This provides for the complement bitline 514′ to beset to logical ‘0’ data.

When the write clock signal 510 is enabled and the data input 908 to bewritten is logical ‘0,’ the charge control circuit 913 generates thecharge control signal 921 that causes a pulse signal 915 is generated bya pulse circuit 917 on the output 912 to connect the complement bitline514′ to Vdd (logical ‘1’ data). After the delay in a delay circuit 916in the charge control circuit 913 expires to allow a complement datainput signal 910 to be asserted onto the complement bitline 514′, thepropagation of the write clock signal 510 will cause the charge storagecircuits 716(0)-716(X) in the charge storage unit 707 to boost thecomplement bitline 514′. This boosts the charge on the complementbitline 514′ to increase the voltage of the complement bitline 514′ toincrease the strength of the PFET access transistors 528(0), 528(1).

FIG. 10 illustrates another exemplary write-assist circuit 1000 providedin a bitline driver 512B to perform a write operation on the memory bitcell 502. The write-assist circuit 1000 includes a bitline positiveboost circuit 1002 configured to boost the voltage of the bitline 514and complement bitline 514′ of the memory bit cell 502 in FIG. 5 toincrease the gate-to-source voltage (V_(GS)) of the PFET accesstransistors 528(0), 528(1) and strengthen the PFET access transistors528(0), 528(1) during a write operation to avoid write contentions.Common components between the bitline driver 512B in FIG. 10 and thebitline driver 512A in FIG. 9 are shown with common element numbers inFIG. 10, and thus will not be re-described here.

With reference to FIG. 10, before a write operation is initiated by thewrite clock signal 510, the complement bitline 514′ will be in afloating state. More specifically, when the write clock signal 510 isnot enabled, the output 907 of the NAND gate 909 in the complementbitline control circuit 903′ will be logical ‘1,’ thereby turning off aPFET 904 and an NFET 906 to leave the complement bitline 514′ in thefloating state. In the charge storage unit 707, the charge storagecircuits 716(0)-716(X) are charged when the complement data input signal910 is a logical ‘1’. The charge in the charge storage circuits716(0)-716(X) will be discharged when the complement data input signal910 is a logical ‘0,” because a logical ‘1’ being asserted on thebitline 514 or complement bitline 514′ is the condition when a writecontention can occur. This condition exists when the complement datainput signal 910 is a logical ‘0’ in FIG. 10, because the inverter 914inverts the complement data input signal 910 onto the complement bitline514′.

In this regard, when the write clock signal 510 is not enabled and thecomplement data input signal 910 is a logical ‘1’, the charge generatingcircuits 714(0)-714(X) in the boost generator circuit 705 are enabledaccording to their respective programmable charge lines b(0)-b(x) tocharge their respective charge storage circuits 716(0)-716(X). A chargeactivation circuit 1005 is provided in the bitline positive boostcircuit 1002 in FIG. 10. The charge activation circuit 1005 isconfigured to receive a discharge control signal 1009 from thecomplement bitline control circuit 903′. The charge activation circuit1005 is configured to selectively couple the charge storage unit 1007 tothe complement bitline 514′ based on the discharge control signal 1009to control the discharge of the charge stored in the charge storage unit1007 onto the complement bitline 514′ to positively boost the voltage onthe complement bitline 514′.

In this regard, in this example, PFETs 1004(0)-1004(X) in the chargeactivation circuit 1005 are enabled to provide a charge path forrespective charge storage circuits 716(0)-716(X) in the charge storageunit 1007 to ground (GND). PFETs 1006(0)-1006(X) in the chargeactivation circuit 1005 are disabled to disconnect the charge path ofthe charge storage circuits 716(0)-716(X) to the complement bitline514′. When the write clock signal 510 is enabled during a writeoperation and the complement data input signal 910 is a logical ‘1’, thecharge generating circuits 714(0)-714(X) are still enabled according totheir respective programmable charge lines b(0)-b(x) to charge theirrespective charge storage circuits 716(0)-716(X). Again, the PFETs1004(0)-1004(X) in the charge activation circuit 1005 are enabled toprovide a charge path of the charge storage circuits 716(0)-716(X) toground (GND). The PFETs 1006(0)-1006(X) in the charge activation circuit1005 are disabled to disconnect the charge path of the charge storagecircuits 716(0)-716(X) to the complement bitline 514′. However, when thewrite clock signal 510 is enabled during a write operation and thecomplement data input signal 910 is a logical ‘0’, a logical ‘1’ isasserted onto the complement bitline 514.′ In this regard, the PFETs1004(0)-1004(X) in the charge activation circuit 1005 are disabled suchthat a charge path does not exist between the charge storage circuits716(0)-716(X) and ground (GND). The PFETs 1006(0)-1006(X) in the chargeactivation circuit 1005 are enabled to connect the charge path of thecharge storage circuits 716(0)-716(X) to the complement bitline 514′ todischarge the charge stored in the charge storage circuits 716(0)-716(X)to the complement bitline 514′, to provide a voltage boost to thecomplement bitline 514′.

Note that although the bitline positive boost circuit 1002 in FIG. 10 isshown as connected to the complement bitline 514′ of the memory bit cell502, a bitline boost circuit 1002 can also be provided and connected tothe bitline 514 of the memory bit cell 502.

Note that it also possible to weaken the respective pull-down NFET526(0) and/or 526(1) of the cross-coupled inverters 520(0) or 520(1) inthe memory bit cell 502, rather than, or in addition to, negativelyboosting the wordline 506 and/or positively boosting the bitline 514 orcomplement bitline 514′ to avoid a write contention. In this regard,FIG. 11 illustrates an exemplary write-assist circuit 1100 configured toweaken the pull-down NFETs 526(0) in the cross-coupled inverter 520(0)in the memory bit cell 502 in FIG. 5 to be weaker than the respectivePFET access transistor 528(0) in response to a write operation to avoidor reduce write contention. In this example, as will be discussed below,the write-assist circuit 1100 is provided in the form of a negativesupply rail positive boost circuit 1102. The negative supply railpositive boost circuit 1102 is configured to positively boost a negativesupply rail 1103 of the pull-down NFETs 526(0) in the cross-coupledinverter 520(0) of the storage circuit 501 of the memory bit cell 502 inthis example.

Note that although the negative supply rail positive boost circuit 1102is shown coupled to the pull-down NFET 526(0) of the inverter 520(0) inFIG. 11, the negative supply rail positive boost circuit 1102 could alsobe provided to be coupled to the pull-down NFET 526(1) of the inverter520(1) in lieu of or in addition to the pull-down NFET 526(0) ofinverter 520(0), if the write contention to be avoided or reduced for awrite operation is also between the pull-down NFET 526(1) and the PFETaccess transistors 528(1).

In this regard, with reference to FIG. 11, the memory bit cell 502 isshown. The negative supply rail positive boost circuit 1102 in thisexample includes a voltage control circuit 1105. The voltage controlcircuit 1105 is configured to control the voltage provided to thenegative supply rail 1103 of the storage circuit 501 based on thewordline enable signal 507 indicating if a write operation is occurringor not. In this regard, the voltage control circuit 1105 includes afirst control circuit 1107. The first control circuit 1107 is configuredto couple a first supply voltage (Vss) to the negative supply rail 1103in response to the wordline enable signal 507 not indicating the writeoperation in this example. Thus, when the write clock signal 510 isdisabled and the wordline enable signal 507 is not enabled, meaning awrite operation is not occurring, an output 1102 of a NAND gate 1104 inthe negative supply rail positive boost circuit 1102 will be logical‘1.’ This will turn on an NFET 1106 in the first control circuit 1107 inthis example to be connected to ground (GND). PFETs 1108, 1110 in asecond control circuit 1109 will not be activated and thus will beturned off. Thus, the pull-down NFET 526(0) in the inverter 520(0) willbe connected through a node 1111 to couple the negative supply rail 1103of inverter 520(0) to ground (GND), which will not weaken the pull-downNFET 526(0). Thus, during read operations for example, the pull-downNFET 526(0) is coupled to ground (GND).

However, in response to a write operation for the memory bit cell 502,the second control circuit 1109 provided in the voltage control circuit1105 is configured to couple a voltage based on the second supplyvoltage (Vdd) to the negative supply rail 1103 in response to thewordline enable signal 507 indicating a write operation. The secondsupply voltage (Vdd) has a higher voltage level than the first supplyvoltage (Vss) in this example. Coupling a voltage based on the secondsupply voltage (Vdd) to the negative supply rail 1103 in response to thewrite operation reduces the gate-to-source voltage (VGs) of thepull-down NFET 526(0) in the inverter 520(0) in the storage circuit 510,and weakens the pull-down NFET 526(0) during a write operation. In thisregard, the write clock signal 510 is enabled causing a pulse signal1113 to be generated from a pulse generator 1115 based on the writeclock signal 510. The wordline enable signal 507 is also enabled for thewrite operation for the memory bit cell 502. Thus, the output 1102 ofthe NAND gate 1104 will be logical ‘0,’ which will turn off the NFET1106 such that the node 1111 is no longer coupled to ground (GND). ThePFET 1108 and PFET 1110 in the second control circuit 1109 will beturned on in a voltage divider arrangement to divide voltage (Vdd)between the node 1111 and ground (GND) to provide a source voltagegreater than the voltage at ground (GND) at the node 1111 and thenegative supply rail 1103 of the pull-down NFET 526(0) in the inverter520(0). This has the effect of reducing the gate-to-source voltage ofthe pull-down NFET 526(0) in the inverter 520(0) to weaken the pull-downNFET 526(0) in response to a write operation to avoid or reduce writecontentions between the PFET access transistor 528(0) and the pull-downNFET 526(0) in this example.

The ratio of the sizes (i.e., resistances) of the PFET 1108 and PFET1100 determines how the voltage (Vdd) will be divided among the PFET1108 and PFET 1100, and thus the voltage at the node 1111. For example,if the PFET 1108 and PFET 1100 are the same size, the voltage (Vdd) willbe approximately split in half (Vdd/2) between the PFET 1108 and PFET1100, providing voltage (Vdd)/2 at the node 1111 to the pull-down NFETs526(0).

FIG. 12 illustrates another exemplary write-assist circuit 1200 alsoconfigured to weaken the pull-down NFET 526(0) in the inverters 520(0)in the memory bit cell 502 in FIG. 5 to be weaker than the respectivePFET access transistors 528(0) in response to a write operation to avoidor reduce a write contention. In this example, as will be discussedbelow, a write-assist circuit 1200 is also provided in the form of anegative supply rail positive boost circuit 1202. The negative supplyrail positive boost circuit 1202 is configured to positively boost anegative supply rail 1103 of the pull-down NFETs 526(0) in thecross-coupled inverter 520(0) of the storage circuit 501 of the memorybit cell 502 in this example. Note that although the negative supplyrail positive boost circuit 1202 is shown coupled to the pull-down NFET526(0) of the inverter 520(0) in FIG. 12, the negative supply railpositive boost circuit 1102 could also be provided to be coupled to thepull-down NFET 526(1) of the inverter 520(1), if the write contention tobe avoided or reduced for a write operation is between the pull-downNFET 526(1) and the PFET access transistor 528(1). Common componentsbetween the memory bit cell 502 and write-assist circuit 1200 in FIGS.11 and 12 are shown with common element numbers, and thus will not bere-described.

In this regard, with reference to FIG. 12, when the write clock signal510 is disabled, the output 1102 of the NAND gate 1104 will be logical‘1.’ A first control circuit 1206 in a voltage control circuit 1204 isprovided. The first control circuit 1206 is configured to couple thefirst supply voltage (VSS) to the negative supply rail 1103 in responseto the wordline enable signal 507 not indicating the write operation inthis example. Thus, when the write clock signal 510 is disabled and thewordline enable signal 507 is not enabled, meaning a write operation isnot occurring, an output 1102 of a NAND gate 1104 in the negative supplyrail positive boost circuit 1102 will be a logical ‘1.’ This will turnon an NFET 1210 in the first control circuit 1206 in this exampleconnected to ground (GND), to connect the node 1111 to ground (GND).Thus, the negative supply rail 1103 of the pull-down NFET 526(0) in theinverter 520(0) will be coupled through the node 1111 to ground (GND),which will not weaken the pull-down NFET 526(0) or 526(1). The PFET 1110in a second control circuit 1208 in the voltage control circuit 1204will not be turned on or activated.

However, when the write clock signal 510 is enabled and the wordlineenable signal 507 is enabled in response to a write operation for thememory bit cell 502, the second control circuit 1208 provided in thevoltage control circuit 1204 is configured to couple a voltage based onthe second supply voltage (Vdd) to the negative supply rail 1103 inresponse to the wordline enable signal 507 indicating a write operation.The second supply voltage (Vdd) has a higher voltage level than thefirst supply voltage (Vss) in this example. Coupling a voltage based onthe second supply voltage (Vdd) to the negative supply rail 1103 inresponse to the write operation reduces the gate-to-source voltage (VGs)of the pull-down NFET 526(0) in the inverter 520(0) in the storagecircuit 501, and weakens the pull-down NFET 526(0) during a writeoperation. In this regard, in this example, the output 1102 of the NANDgate 1104 will be logical ‘0,’ which will turn off the NFET 1210 in thefirst control circuit 1206. The NFET 1210 will be turned off and thePFET 1110 in the second control circuit 1208 will be activated or turnedon. Thus, voltage (Vdd) will be provided by the PFET 1110 in the secondcontrol circuit 1208 to the node 1111 and to the negative supply rail1103 of the pull-down NFET 526(0) in the inverter 520(0). In thisexample, to allow the voltage provided to the node 1111 and thus thenegative supply rail 1103 based on the second supply voltage (Vdd) to bevariably controlled, another NFET 1212 is provided in the second controlcircuit 1208 in this example. The NFET 1212 is coupled to ground (GND)and is controlled by a voltage of a bias input signal (BIAS) in avoltage divider arrangement with the PFET 1110 to provide a voltagesignal greater than ground (GND) to the negative supply rail 1103 of thepull-down NFET 526(0) in the inverter 520(0) to weaken the pull-downNFET 526(0). The bias input signal (BIAS) controls the resistance of theNFET 1212 and thus the amount of voltage (Vdd) divided between the PFET1110 and the NFET 1212 at node 1111. The voltage at the node 1111 isprovided to the negative supply rail 1103.

The PFET write port bit cells and write-assist circuits disclosed hereinto avoid write conditions in the PFET read/write port bit cellsaccording to aspects disclosed herein, may be provided in or integratedinto a memory in any processor-based device. Examples, withoutlimitation, include a set top box, an entertainment unit, a navigationdevice, a communications device, a fixed location data unit, a mobilelocation data unit, a mobile phone, a cellular phone, a computer, aportable computer, a desktop computer, a personal digital assistant(PDA), a monitor, a computer monitor, a television, a tuner, a radio, asatellite radio, a music player, a digital music player, a portablemusic player, a digital video player, a video player, a digital videodisc (DVD) player, and a portable digital video player.

In this regard, FIG. 13 illustrates an example of a processor-basedsystem 1300 that can employ memory systems that include PFET write portbit cells employing write-assist circuits disclosed herein to avoidwrite contentions in the PFET write port bit cells according to aspectsdisclosed herein. In this example, the processor-based system 1300includes one or more central processing units (CPUs) 1302, eachincluding one or more processors 1304. The CPU(s) 1302 may have cachememory 1306 coupled to the processor(s) 1304 for rapid access totemporarily stored data. The cache memory 1306 can employ PFET writeport bit cells 1308, including the PFET write port memory bit cell 502illustrated in FIG. 5. The CPU(s) 1302 is coupled to a system bus 1310and can intercouple master and slave devices included in theprocessor-based system 1300. As is well known, the CPU(s) 1302communicates with these other devices by exchanging address, control,and data information over the system bus 1310. For example, the CPU(s)1302 can communicate bus transaction requests to a memory controller1312 in a memory system 1314 as an example of a slave device. Althoughnot illustrated in FIG. 13, multiple system buses 1310 could beprovided, wherein each system bus 1310 constitutes a different fabric.In this example, the memory controller 1312 is configured to providememory access requests to a memory array 1316 in the memory system 1314.The memory array 1316 can also include PFET write port bit cells 1318that include read assist circuits.

Other devices can be connected to the system bus 1310. As illustrated inFIG. 13, these devices can include the memory system 1314, one or moreinput devices 1320, one or more output devices 1322, one or more networkinterface devices 1324, and one or more display controllers 1326, asexamples. The input device(s) 1320 can include any type of input device,including but not limited to input keys, switches, voice processors,etc. The output device(s) 1322 can include any type of output device,including but not limited to audio, video, other visual indicators, etc.The network interface device(s) 1324 can be any devices configured toallow exchange of data to and from a network 1328. The network 1328 canbe any type of network, including but not limited to a wired or wirelessnetwork, a private or public network, a local area network (LAN), a widelocal area network (WLAN), and the Internet. The network interfacedevice(s) 1324 can be configured to support any type of communicationsprotocol desired.

The CPU(s) 1302 may also be configured to access the displaycontroller(s) 1326 over the system bus 1310 to control information sentto one or more displays 1330. The display controller(s) 1326 sendsinformation to the display(s) 1330 to be displayed via one or more videoprocessors 1332, which process the information to be displayed into aformat suitable for the display(s) 1330. The display(s) 1330 can includeany type of display, including but not limited to a cathode ray tube(CRT), a liquid crystal display (LCD), a plasma display, etc.

A non-transitory computer-readable medium, such as the memory system1314 in FIG. 13 as a non-limiting example, may also have stored thereoncomputer for an integrated circuit (IC). The computer data may beprovided in the form of a library cell 1334 in a cell library in thisexample. The computer data, when executed, can cause the CPU 1302 tostore the library cell 1334 for an IC or IC design.

In this regard, in this example, the library cell 1334 comprises alibrary memory bit cell 1336, which may be a library cell based on thedesign of any PFET write port memory bit cells discussed herein. Asdiscussed above, the PFET write port memory bit cells comprise a memorysystem 1314 comprising a memory bit cell configured to store data inresponse to a write operation, the memory bit cell comprising one ormore PFET access transistors each comprising a gate configured to beactivated by a wordline in response to the write operation. The PFETwrite port memory bit cells also comprise a wordline negative boostcircuit coupled to the wordline, the wordline negative boost circuitconfigured to negatively boost a voltage on the wordline to negativelyboost a voltage on the gate of the one or more PFET access transistorsin response to the write operation.

The library memory bit cell 1336 may be a library cell based on thedesign of a write port memory bit cell that comprises a memory systemcomprising a memory bit cell configured to store data in response to awrite operation. The memory bit cell comprises one or more PFET accesstransistors each comprising a gate configured to be activated by awordline in response to the write operation, and an access nodeconfigured to receive data from a bitline. The memory system alsocomprises a bitline positive boost circuit coupled to the bitline, thebitline positive boost circuit configured to positively boost a voltageon at least one bitline in response to the write operation.

The library memory bit cell 1336 may be a library cell based on thedesign of a write port memory bit cell that comprises a memory systemcomprising a memory bit cell configured to store data in a storagecircuit in response to a write operation. The memory bit cell comprisesthe storage circuit, which comprises a positive supply rail and anegative supply rail, one or more inverters each comprising a pull-upPFET coupled to the positive supply rail and a pull-down NFET coupled tothe negative supply rail, and one or more PFET access transistorscoupled to the storage circuit and configured to pass data from at leastone bitline to the storage circuit in response to activation of awordline in the write operation. The memory system also comprises anegative supply rail positive boost circuit coupled to the negativesupply rail of at least one inverter among the one or more inverters,the negative supply rail positive boost circuit configured to positivelyboost a voltage on the negative supply rail to weaken the pull-down NFETin the one or more inverters in the storage circuit in response to thewrite operation.

Note that the use of PFET and NFET in this disclosure can includePMOSFETs and NMOSFETs that are metal oxide semiconductors (MOSs). ThePFETs and NFETs discussed herein can include other types of oxide layersother than metal. Also note that any of the assist circuits disclosedherein can be provided for either or both of the bitline and complementbitline of the bit cells disclosed herein.

Those of skill in the art will further appreciate that the variousillustrative logical blocks, modules, circuits, and algorithms describedin connection with the aspects disclosed herein may be implemented aselectronic hardware, instructions stored in memory or in anothercomputer-readable medium and executed by a processor or other processingdevice, or combinations of both. The master and slave devices describedherein may be employed in any circuit, hardware component, integratedcircuit (IC), or IC chip, as examples. Memory disclosed herein may beany type and size of memory and may be configured to store any type ofinformation desired. To clearly illustrate this interchangeability,various illustrative components, blocks, modules, circuits, and stepshave been described above generally in terms of their functionality. Howsuch functionality is implemented depends upon the particularapplication, design choices, and/or design constraints imposed on theoverall system. Skilled artisans may implement the describedfunctionality in varying ways for each particular application, but suchimplementation decisions should not be interpreted as causing adeparture from the scope of the present disclosure.

The various illustrative logical blocks, modules, and circuits describedin connection with the aspects disclosed herein may be implemented orperformed with a processor, a Digital Signal Processor (DSP), anApplication Specific Integrated Circuit (ASIC), a Field ProgrammableGate Array (FPGA) or other programmable logic device, discrete gate ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. A processormay be a microprocessor, but in the alternative, the processor may beany conventional processor, controller, microcontroller, or statemachine. A processor may also be implemented as a combination ofcomputing devices, e.g., a combination of a DSP and a microprocessor, aplurality of microprocessors, one or more microprocessors in conjunctionwith a DSP core, or any other such configuration.

It is also noted that the operational steps described in any of theexemplary aspects herein are described to provide examples anddiscussion. The operations described may be performed in numerousdifferent sequences other than the illustrated sequences. Furthermore,operations described in a single operational step may actually beperformed in a number of different steps. Additionally, one or moreoperational steps discussed in the exemplary aspects may be combined. Itis to be understood that the operational steps illustrated in the flowchart diagrams may be subject to numerous different modifications aswill be readily apparent to one of skill in the art. Those of skill inthe art will also understand that information and signals may berepresented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples and designs described herein, but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. A memory system, comprising: a memory bit cellconfigured to store data in response to a write operation, the memorybit cell coupled between a positive supply rail and a negative supplyrail and configured to be powered by a voltage source supplying a supplyvoltage to the positive supply rail and a ground voltage to the negativesupply rail; the memory bit cell comprising one or more P-typeField-Effect Transistor (PFET) access transistors each comprising a gateconfigured to be activated by a wordline in response to the writeoperation; and a wordline negative boost circuit coupled to thewordline, the wordline negative boost circuit configured to negativelyboost a voltage on the wordline below the ground voltage to negativelyboost a voltage on the one or more gates of the one or more PFET accesstransistors in response to the write operation.
 2. The memory system ofclaim 1, wherein: the one or more PFET access transistors each furthercomprising a source; and the wordline negative boost circuit isconfigured to negatively boost a voltage across the gate and the source(V_(GS)) of the one or more PFET access transistors in response to thewrite operation, by negatively boosting the voltage on the wordline inresponse to the write operation.
 3. The memory system of claim 1,wherein the memory bit cell further comprises: a storage circuit; andthe one or more PFET access transistors coupled to the storage circuitconfigured to pass data from at least one bitline to the storage circuitin response to activation of the wordline in the write operation.
 4. Thememory system of claim 3, wherein the wordline negative boost circuit isconfigured to negatively boost the voltage on the wordline below theground voltage in response to the write operation, to assisttransferring the data from the at least one bitline to the storagecircuit.
 5. The memory system of claim 3, wherein: the storage circuitcomprises one or more inverters each comprising a pull-up PFET coupledto a pull-down N-type Field-Effect transistor (NFET); and the wordlinenegative boost circuit is configured to negatively boost the voltageacross the gate and a source (V_(GS)) of the one or more PFET accesstransistors to reduce or prevent discharge of the pull-down NFET, inresponse to the write operation.
 6. The memory system of claim 3,wherein: the one or more PFET access transistors comprises a first PFETaccess transistor coupled to the storage circuit and a second complementPFET access transistor coupled to the storage circuit; the first PFETaccess transistor configured to pass a first voltage from a bitline tothe storage circuit in response to activation of the wordline in thewrite operation; and the second complement PFET access transistorconfigured to pass a second voltage complementary to the first voltagefrom a complement bitline to the storage circuit in response toactivation of the wordline in the write operation.
 7. The memory systemof claim 6, wherein: the storage circuit is comprised of a storage nodeand a complement storage node; the first PFET access transistor isconfigured to pass the first voltage from the bitline to the storagenode in response to activation of the wordline in the write operation;and the second complement PFET access transistor is configured to passthe second voltage complementary to the first voltage from thecomplement bitline to the complement storage node in response toactivation of the wordline in the write operation.
 8. The memory systemof claim 1, wherein the wordline negative boost circuit is configured tonegatively boost the voltage on the wordline below the ground voltage inresponse to a wordline enable signal indicating the write operation. 9.The memory system of claim 3, further comprising a bitline positiveboost circuit coupled to the at least one bitline, the bitline positiveboost circuit configured to positively boost a voltage on the at leastone bitline in response to the write operation.
 10. The memory system ofclaim 3, further comprising a negative supply rail positive boostcircuit coupled to the negative supply rail of the storage circuit, thenegative supply rail positive boost circuit configured to positivelyboost a voltage on the negative supply rail, to further assist intransferring the data from an access node to the storage circuit inresponse to the write operation.
 11. The memory system of claim 1,comprising a plurality of the memory bit cells each configured to storedata in response to a write operation, each memory bit cell among theplurality of memory bit cells comprising one or more PFET accesstransistors each comprising a gate configured to be activated by awordline in response to the write operation; and the wordline negativeboost circuit coupled to the wordline configured to activate the gate ofeach of the plurality memory bit cells, the wordline negative boostcircuit configured to negatively boost the voltage on the wordline belowthe ground voltage to negatively boost a voltage on the one or moregates of the one or more PFET access transistors in response to thewrite operation.
 12. The memory system of claim 11, wherein theplurality of the memory bit cells are disposed in a memory bit cell rowof a memory array.
 13. The memory system of claim 1 provided in aprocessor-based system.
 14. The memory system of claim 13, wherein theprocessor-based system is comprised of a central processing unit(CPU)-based system.
 15. The memory system of claim 1 integrated into asystem-on-a-chip (SoC).
 16. The memory system of claim 1 integrated intoa device selected from the group consisting of: a set top box, anentertainment unit, a navigation device, a communications device, afixed location data unit, a mobile location data unit, a mobile phone, acellular phone, a computer, a portable computer, a desktop computer, apersonal digital assistant (PDA), a monitor, a computer monitor, atelevision, a tuner, a radio, a satellite radio, a music player, adigital music player, a portable music player, a digital video player, avideo player, a digital video disc (DVD) player, and a portable digitalvideo player.
 17. A memory system, comprising: a means for storing datafor a write operation, the means for storing data comprising one or moreP-type Field-Effect Transistor (PFET)-based access means each comprisinga gate for being activated by a wordline in response to the writeoperation; and a means for negatively boosting a voltage on the wordlinebelow a ground voltage in response to the write operation.
 18. A methodof writing data to a memory bit cell, comprising: charging at least onebitline coupled to an access node of one or more P-type Field-EffectTransistor (PFET) access transistors in a memory bit cell with data inresponse to a write operation, the memory bit cell coupled between apositive supply rail and a negative supply rail and configured to bepowered by a voltage source supplying a supply voltage to the positivesupply rail and a ground voltage to the negative supply rail; activatinga wordline coupled to a gate of the one or more PFET access transistorsin response to the write operation to transfer the data from an accessnode to a storage circuit; and negatively boosting a voltage on thewordline below the ground voltage in response to the write operation toassist transferring the data from the access node to the storage circuitin response to the write operation.
 19. The method of claim 18,comprising negatively boosting the voltage on the wordline below theground voltage to boost a voltage across the gate and a source (V_(GS))of the one or more PFET access transistors in response to the writeoperation by negatively boosting the voltage on the wordline in responseto the write operation.
 20. The method of claim 18, comprisingnegatively boosting the voltage on the wordline below the ground voltageto boost a voltage across the gate and a source (V_(GS)) of the one ormore PFET access transistors in response to the write operation, toreduce or prevent discharge of a pull-down N-type FET (NFET) in thestorage circuit, in response to the write operation to assisttransferring the data from the access node to the storage circuit. 21.The method of claim 18, wherein: charging the at least one bitlinecomprises: charging a bitline coupled to a first access node of a firstPFET access transistor in the memory bit cell with first data inresponse to the write operation; and charging a complement bitlinecoupled to a second access node of a second PFET access transistor inthe memory bit cell with second data complementary to the first data inresponse to the write operation; activating the wordline comprises:activating the wordline coupled to a gate of the first PFET accesstransistor in response to the write operation to transfer the first datafrom the first access node to the storage circuit; and activating thewordline coupled to a gate of the second PFET access transistor inresponse to the write operation to transfer the second data from thesecond access node to the storage circuit; and negatively boosting thevoltage comprises negatively boosting the voltage on the wordline belowthe ground voltage in response to the write operation to assisttransferring the first data from the first access node to the storagecircuit and transferring the second data from the second access node tothe storage circuit.
 22. The method of claim 21, comprising: activatingthe wordline coupled to the gate of the first PFET access transistor inresponse to the write operation to transfer the first data from thefirst access node to a first storage node in the storage circuit; andactivating the wordline coupled to the gate of the second PFET accesstransistor in response to the write operation to transfer the seconddata from the second access node to a second storage node in the storagecircuit.
 23. The method of claim 18, further comprising positivelyboosting a voltage on the negative supply rail of the storage circuit inresponse to a write operation, to further assist in transferring thedata from the access node to the storage circuit in response to thewrite operation.
 24. A non-transitory computer-readable medium havingstored thereon computer data for an integrated circuit (IC), theintegrated circuit comprising: a memory bit cell configured to storedata in response to a write operation, the memory bit cell comprisingone or more P-type Field-Effect Transistor (PFET) access transistorseach comprising a gate configured to be activated by a wordline inresponse to the write operation; and a wordline negative boost circuitcoupled to the wordline, the wordline negative boost circuit configuredto negatively boost a voltage on the wordline below a ground voltage tonegatively boost a voltage on the gate of the one or more PFET accesstransistors in response to the write operation.
 25. The memory system ofclaim 1, wherein the wordline negative boost circuit is configured tonegatively boost the voltage on the wordline below the ground voltage inresponse to activation of a wordline enable signal and a write clocksignal indicating the write operation.
 26. The memory system of claim17, wherein the means for negatively boosting the voltage on thewordline is configured to negatively boost the voltage on the wordlinebelow the ground voltage in response to activation of a wordline enablesignal and a write clock signal indicating the write operation.
 27. Themethod of claim 18, wherein negatively boosting the voltage on thewordline below the ground voltage comprises negatively boosting thevoltage on the wordline below the ground voltage in response toactivation of a wordline enable signal and a write clock signalindicating the write operation.
 28. The non-transitory computer-readablemedium of claim 24, wherein the wordline negative boost circuit isconfigured to negatively boost the voltage on the wordline below theground voltage in response to activation of a wordline enable signal anda write clock signal indicating the write operation.